Discovery Bay agal bloom

Photo by Tony Kukulich

A harmful agal bloom has been identified around the marina in Discovery Bay, Calif., as seen Tuesday, June 16, 2020.

Water samples gathered in Discovery Bay on Thursday, June 4, have tested positive for dangerous levels of cyanotoxins.

Samples were gathered from multiple locations, but those with high levels came from Windward and Clipper bays and the water in front of the Discovery Bay Yacht Club (DBYC). 

“I’m a little bit surprised — it hasn’t been super hot yet,” said Joe Doser, supervising environmental health specialist for Contra Costa County. “We’ve had a few hot days … but the results are pretty significant — way above the danger level.”

Blue-green algae (BGA) grow in warm, stagnant and nutrient-rich shallows, blooming and releasing cyanobacteria, a toxin harmful to people and pets. The blooms are distinguished by a green, blue-green, white or brown scum that floats on the surface or suspends in the water. The blooms dissipate with fall’s cooler temperatures and rain. 

The county received the results from the samples on June 10 and released the information, advising local entities to post signage detailing ways to stay safe by avoiding the blooms. Harmful algae blooms (HAB) generally occur on the edges of bays or near docks and do not affect recreational boating traffic or fast-moving waterways. Those looking to swim off their docks should check their water carefully for signs of HAB. 

Discovery Bay is not the only water community suffering from HAB. Samples taken at Oakley’s Big Break Regional Park this month also tested positive for cyanotoxins at dangerous levels, as have multiple bodies of water across the state. 

Some residents have taken issue with the samples, as they were gathered right next to docks, rather than in the middle of the bays. They argued that the samples do not paint an accurate picture of the water, as algae along the rocky edges of the town’s bays is able to survive almost year-round. 

“It’s unfortunate these samples were taken next to the levee where the scum collects, instead of a minimum of 100 feet away from the docks where it is a better representation of the water quality in the bays,” Discovery Bay resident Jim Mattison said. “Normally, we take monthly samples from the boat, but because of COVID-19, the state has guidelines for now that do not allow their staff in close proximity with others.”

Mattison also theorized the algae will continue to worsen until the state regulates the amount of nitrates, phosphates and other toxins washed into the Delta by local farmers. He noted those chemicals feed the HAB.  

Although the Town of Discovery Bay doesn’t own municipal waterfront property, it created warnings signs for residents who wish to post them.

“We don’t have any district property that I’m aware of that is on the waterways where this is occurring,” said Discovery Bay general manager Mike Davies. “As information becomes available to the town, we have links to it on the website, so residents should check it frequently, utilize the links and get the most updated information that we as a town have on what’s going on with the algae situation.”

Residents should contact the county if they see the algae blooming near their docks, as the town itself has neither the funding nor the authority to deal with it.

Normally, HAB becomes an issue after weeks of high temperatures and dangerous levels of cyanotoxins aren’t reached until August. No one is sure why the algae have bloomed so early, but there has been speculation as to the cause — one change in Discovery Bay waters this year was the Department of Boating and Waterways’ delay in treating invasive weeds. In previous years, weed treatments began in March, but due to delays caused by the coronavirus, spraying did not begin this year until early June. 

Another possibility is an increase in fertilizers being washed into the delta as more residents took up gardening during the shelter-in-place. These are simply speculations and not yet backed by scientific data.

One local organization attempting to find a solution to the HAB is the Discovery Bay Community Foundation. The foundation, along with the Central Valley Water Board, has secured an $80,000 grant to conduct a load study of the water in and around Discovery Bay. The hope is the study will shed more light on how and why the algae has grown exponentially and will be conducted once COVID-19 restrictions are eased.

The harbormaster at the Discovery Bay Marina has posted signs from the county warning people of the algae.  

Human exposure to water containing toxic harmful algal blooms, for example by direct body contact or ingestion, can result in a number of symptoms including the following:

• Rashes

• Eye, nose, mouth or throat irritation

• Allergic reactions

• Headache

• Gastrointestinal upset, such as abdominal pain, nausea, vomiting and diarrhea

Pets are especially susceptible, as they can ingest the toxins when licking their bodies.

More information can be found on the Town of Discovery Bay’s website at, the Contra Costa Environmental Health website at, or the State of California’s HAB portal at

If you have questions or concerns, contact the HAB Hotline by calling 1-844-729-6466 or emailing